1. Field of the Invention
The present invention relates to a solid-state image pickup device applied to an image input system or the like, and particularly to an MOS type solid-state image pickup device. Further, the present invention relates to an image input device having an MOS type solid-state image pickup device mounted therein.
2. Description of the Related Art
Recently, a digital camera, a PC camera, an optical mouse, a portable TV telephone, etc. have been developed as an image input system using a solid-state image pickup device. These devices have been required to be low in driving voltage and power consumption not only from the image quality, but also from the viewpoint of battery using time and compact-size. CCD sensors have difficulties to satisfy these requirements, and thus MOS-type solid-state image pickup devices have been more frequently used. The MOS type solid-state image pickup device has advantages that it is driven with a single power source, has low power consumption and is designed in a system-on-chip form, and also it has a large degree of freedom for read-out. For example, only a part of an image can be output (cut-out operation), and information of an image can be output skip by skip (thin-out operation).
FIG. 9 shows the overall construction of a conventional MOS type solid-state image pickup device.
An MOS type solid-state image pickup device 1 comprises a sensor portion 2 having many unit pixels arranged in a matrix form, each unit pixel comprising a photodiode for performing photoelectric conversion and an MOS switch, a vertical scan circuit 3 and a horizontal scan circuit 4 for driving the sensor portion 2, a CDS (correlated double sampling)/signal holding circuit 5 for receiving signals of pixels of one line of the sensor portion 2, an output amplifier 6, a timing generating circuit 7 for generating pulses to operate the vertical scan circuit 3, the horizontal scan circuit 4, the CDS/signal holding circuit 5 and the output amplifier 6, and a serial interface 8.
In the sensor portion 2, each of vertical selecting lines 10 from the vertical scan circuit 3 is commonly connected to the pixels of each line to supply vertical scan pulses φV [φV1, φV2, . . . φVn] from the vertical scan circuit 3 to the pixels of the respective lines through the vertical selection lines 10 at the same time. Further, each of vertical signal lines is commonly connected to the pixels of each column, and each vertical signal line 11 is connected to a horizontal signal line 12 through the CDS/signal holding circuit 5. The horizontal signal line 12 is connected to the input side of the output amplifier 6. The horizontal scan circuit 4 supplies the horizontal switches of the CDS/signal holding circuit 5 with horizontal scan pulses φH [φH1, φH2, . . . , φHn] to select a pixel signal from the CDS/signal circuit 5 and output it to the horizontal signal line 12.
The serial interface 8 is supplied with serial data from the external. Further, a synchronous signal and a clock signal from the external are supplied to the serial interface 8 and the timing generating circuit 7.
In this CMOS type solid-state image pickup device 1, the serial interface 8 receives data from the external, and the operation of the timing generating circuit 7 is controlled in accordance with the data. In the timing generating circuit 7, driving pulses for driving the vertical scan circuit 3, the horizontal scan circuit 4, the CDS/signal holding circuit 5 and the output amplifier 6 in accordance with the data are generated and output to the respective parts. In the sensor portion 2, the scan operation is carried out by the vertical scan circuit 3, that is, the pixel lines are successively selected by the vertical selection pulses φV [φV1, φV2, . . . φVn] from the vertical scan circuit 3 and the signals of the pixels of each line thus selected (scanned) are output through the corresponding vertical signal line 11 to the CDS/signal holding circuit 5.
The CDS/signal holding circuit receives signals of one line, and holds signals achieved by subtracting offset components (corresponding to a fixed pattern noise component) inherent to the respective pixels from the signals of the line concerned. The horizontal switches are successively switched on by the horizontal scan pulses φH [φH1, φH2, . . . , φHn] from the horizontal scan circuit 4, and the signals of the pixels of one line held in the CDS/signal holding circuit 5 are successively read out through the horizontal signal line 12 to the output amplifier 6. The output amplifier 6 amplifies these signals and outputs them to an output terminal tout as analog signals.
The power consumption of the MOS type solid-state image pickup device 1 as described above is equal to about one fifth of that of a CCD solid-state image pickup device, however, the power consumption thereof is required to be further reduced when it is installed in portable equipment. Further, as the number of pixels increases and the output rate (that is, the driving frequency of the horizontal scan circuit) increases, the power consumption is also increased.
Considering the power consumption of the MOS type solid-state image pickup device, it has power consumption equal to or less than 1/10 of that of CCD pixels in a pixel portion, and this value is almost negligible. Further, the power consumption is also relatively small in digital portions (the serial interface 8, the timing generating circuit 7, the vertical scan circuit 3, the horizontal scan circuit 4, etc.), and the output amplifier 6 of the analog circuit has the highest power consumption. Particularly, as the number of pixels increases, the driving frequency increases, so that the frequency characteristic of the output amplifier 6 (hereinafter referred to as “f-characteristic”) must be increased. The increase of the f-characteristic in the analog circuit causes bias current to be increased, and thus the power consumption is more and more increased.
Further, as the number of pixels increases and thus the output rate is also increased, random noises of the output circuit are intensified.
Besides, as the overall power consumption is increased, the image pickup chip is heated to induce thermal current, and the thermal current thus induced enters the photodiodes of the sensor portion, resulting in increase of noise current (so-called dark current). Accordingly, the noise current is increased only around the output amplifier.